Telecommunication networks are networks that allow communication of information over a distance, and often a great distance, via the use of electromagnetic signals. Telecommunication networks based on propagation of electronic signals through electrical conductors have been available since the time of the telegraph. Telecommunication networks based on propagation of optical signals have been a more recent development. Regardless, the invention and improvement in the ability of human beings to communicate over distances, often great distances, have presented a paradigm shift in human interaction, greatly benefiting human civilization.
Conventional telecommunications technology represents the culmination of decades, and in some cases, centuries of human thought and collaboration. At present, sophisticated optical communication networks are capable of transmitting tens of trillions of bits of information per second over a single optical fiber spanning from a few kilometres to a few thousands of kilometres. Optical networks generally exceed the bandwidth capabilities of wired copper networks. Accordingly, optical networks often provide optical backbones at the core of the telecommunication networks.
Typically, these core optical networks use Dense Wavelength Division Multiplexed (DWDM) optical systems in which optical channels (referred to as “DWDM channels”) are allocated by frequency range. These optical systems employ equipment in different sites of the network, each site composed of network elements that constitute the physical layer of the network. Conventional optical networks are often mesh networks with protection and restoration capabilities, and in which there are Reconfigurable Optical Add/Drop Multiplexers (ROADMs) at the nodes of the mesh network.
Optical networks are often configured and provisioned through a “network management system”, which allows one or more people in a network operations center to change the configuration of the network, to monitor the activity and performance of the network, and so forth.
The ROADMs are typically multi-degree ROADMs, which means that there are different possible paths starting and ending from each multi-degree ROADM node. The number of degrees is the number of paths.
A second type of node is in-line amplifiers (ILA). These in-line amplification nodes may use Erbium-doped fiber amplifiers (EDFAs) and can also use Raman amplification (alone or in combination with EDFAs).
Raman amplification can be used to extend the optical reach (by improving the Optical Signal to Noise Ratio—OSNR) and/or expanding the optical bandwidth of the DWDM optical systems and therefore the capacity.
A third type of node is emerging today and is named optical transport network (OTN) switches. OTN switching are nodes with optoelectronic conversion. These are not all-optical nodes like ILA and ROADM nodes.
Telecommunication networks started because of voice traffic needs. They started based on the provisioning of circuits in order to put in communications two sites, A and B. The circuit was bidirectional, which means A talks to B and B talks to A. The capacity in each direction is the same.
This has been true from the 64 kbit/s circuit up to the high-speed/high-capacity channels in the different hierarchies of optical networks: Sonet/SDH, OTN, etc.
It is known that now data traffic has overpassed voice traffic in terms of volume. Data traffic is often based on networks using Internet Protocol (IP).
Data traffic is sometimes bidirectional but can also be unidirectional. Bidirectional means that the same capacity is required in both directions between the two sites of communications. Unidirectional means traffic is going only in one direction (or essentially in one direction, as there is the possibility of the need for a small capacity in the other direction in order to acknowledge to the emitting site that the flow of communications is going well).
An example of the need for unidirectional traffic is the copy of a database from one site to another site. Bidirectional and unidirectional traffic types are two extremes, and intermediate cases are also possible. Therefore, one could characterize traffic by its asymmetry.
Despite the emergence of asymmetric traffic, optical networks conventionally have optical circuits provisioned in a bidirectional fashion. This is in adherence to ITU-T standards (see website itu.org). One fundamental object in optical networks provisioned in accordance with these standards continues to be the OCh element (Optical Channel). This OCh element continues to be the basic element in the OTH (Optical Transport Hierarchy). The OCh element is a bidirectional circuit (by definition in the standards).